In recent years, a piezoelectric transformer is adopted in an inverter for a back light of liquid crystal panel, a DC/DC converter or the like to realize a small-size and thin-type electric circuit.
FIG. 6 is a perspective view showing a conventional, general Rosen-type piezoelectric transformer. The Rosen-type piezoelectric transformer is an element in which ceramics of lead titanate zirconate (PZT)-based material is, for example, used as material. As shown in FIG. 6, in an input part 21 on the left half of a piezoelectric transformer 20, an input electrode 22 with conductors laminated in the thickness direction is provided, and a piezoelectric element between the input electrodes is subjected to polarization processing in the thickness direction. Furthermore, an output electrode 24 is provided on an end face of an output part 23 on the right half of the piezoelectric transformer 20 by, for example, silver baking or the like, and the piezoelectric element of the output part 23 is subjected to the polarization processing in the longitudinal direction.
Here, the input part corresponds to a part of a piezoelectric transformer where input electrodes are provided in the thickness direction and the piezoelectric element between the input electrodes is polarized in the thickness direction, and the output part corresponds to a part of the piezoelectric transformer where the output electrode is provided at the end part and the piezoelectric element between the output electrode and the input part is polarized in the longitudinal direction.
When an AC voltage with frequency nearly equal to a mechanical resonance frequency in the longitudinal direction is applied between the input electrodes of such a general Rosen-type piezoelectric transformer, a strong mechanical vibration is excited in the longitudinal direction of this piezoelectric transformer, and a high voltage is generated at the output electrode due to piezoelectric effect.
Recently, accompanying the trend toward small sized piezoelectric transformer, a center drive type piezoelectric transformer without occurrence of distorted wave has been developed (for example, patent document 1 (Japanese Patent Bulletin No. 3119154)). In this center drive type piezoelectric transformer, there is provided an input part with input electrodes laminated in the central part, and a piezoelectric element between the input electrodes is polarized in the thickness direction. Output parts on both sides of the central part are polarized respectively along the longitudinal direction, and at both end faces, output electrodes are provided by, for example, silver baking or the like.    [patent document 1] Japanese Patent Bulletin No. 3119154    [patent document 2] Japanese Patent Application Laid-Open No. 2000-150981    [patent document 3] Japanese Patent Application Laid-Open No. 2002-305332